A large form of DNA polymerase III has been identified. This form contains the core polymerase, which has a mol. wt. of 165,000 daltons. The large form contains, in addition, cofactors (CoPol-3) which gives the complex a mol. wt. of over 450,000 daltons. The complex is involved in DNA replication. The mode of DNA replication, using this purified form of enzyme, in vitro, will be studied. The product DNA will be analyzed both biochemically and genetically. During phage SPP-1 infection, the core enzyme remains intact but a new complex is formed, which recognizes only phage DNA. The phage specific cofactor synthesis is under the control of a single gene of about 500 nucleotides long. The gene has been mapped. The fragment containing the gene has been identified by restriction enzyme EcoRI cleavage of the phage DNA. The mode of Pol-3 modification by the phage gene will be analyzed in an in vitro system developed in our laboratory. BIBLIOGRAPHIC REFERENCE: In vivo DNA synthesis in Bacillus subtilis. J.J. Andersen and A.T. Ganesan J. Mol. Biol., 1976, in press.